Don't kid yourselves into thinking you are "distributing the load" over
two or more thrust bearings. It is almost impossible to do unless the
tower is being compressed as you add antennas.
If you mount the mast in the rotator and add antennas and then add the
top thrust bearing, how is it going to be pre loaded with some of the
weight? Won't happen unless the tower compresses or the mast shrinks.
A true thrust bearing has a tapered bearing like that of the front wheel
bearings on a car. Or it has sizable top and bottom plates if it is ball
bearing. A bearing that is designed for a horizontal shaft is not a
thrust bearing just because you mount it in the vertical position.
The top bearing may be fine for lateral forces but it is going to do
little for downward weight in addition to the rotator support unless the
rotator is clamped after the weight is applied to the top bearing. Then
it will support all the weight and very little will be supported by the
rotator.
73
Gary k4FMX
ersmar@comcast.net wrote:
> TT:
>
> First of all, the industrial bearings some of us use atop our towers are
> intended for mounting with the mast horizontal, such as in manufacturing
> environments. In those instances, the bearings allow the shaft they support
> to rotate with zero friction against the shaft itself; the bearings and
> lubricant inside the race(s) provide a low-friction means of rotation. To
> fully realize the low-friction benefits of this device, the shaft must be
> mechanically attached to the rotating collar via a setscrew or screws.
> Otherwise, the shaft will merely rotate around within the collar and
> encounter damaging, or potentially destructive, friction.
>
> When we hams take such a bearing and mount it with the axis of rotation
> vertically, as on top of a tower, the same friction risks apply, albeit to a
> lesser extent. That is, unless the mast is mechanically attached to the
> rotating collar, it could spin inside the collar, causing a small amount of
> friction wear on its surface - same as if the mast were inside a Rohn pointy
> top section.
>
> Granted, the potential frictional wear and tear on our masts is minimal
> compared to the friction encountered on a rotating shaft in an industrial
> application, i.e., <1000 RPY (revs per year) in Amateur work vs 100's to
> 1000+ RPS (revs per sec) in industrial useage. NTL, we (some of us at least)
> still install such bearings to provide lateral support for our masts during
> high winds while still allowing the masts to turn freely.
>
> I would not rely on one setscrew (typical of amateur bearings and some
> industrial bearings) as the sole support for my mast and antenna dead weight.
> This situation could occur when replacing a rotator on a tower that has but
> one bearing plate in place - at the tower top. In this case the friction of
> that setscrew (3/16 inch max diameter?) is the only force keeping the mast
> away from my busy hands/wrists/arms on the rotator plate below. Not a good
> idea (see yesterday's postings on this subject.)
>
> Similarly, we seem to tell all who ask that the rotator is designed to
> operate with all of the mast/antenna weight on it and on it alone. This
> might be within the limits of the rotator. But we forget about the safe load
> limits of the rotator's supporting shelf. As I've written here before
> (yesterday), my Trylon's specs show 300# as the dead weight limit of each of
> its rotator/bearing mounting plates. My 23 foot CM mast weighs about 110#;
> my Bencher around 75#; other antennas and coax maybe 20#; rotator itself 10#.
> Total weight on my lower (rotator) shelf is 215#, below the 300# limit.
>
> However, I chose to distribute this load over two plates as follows. I
> installed the top GP15 and D40 antennas plus coax while the mast was barely
> through the top of the tower. I (actually N3RR and I) hoisted this
> mast/antenna/coax assembly up to the final position above the rotator plate
> and installed the rotator underneath. Then we lowered the mast onto the
> rotator and tightened the rotator's u-bolts against the mast. Thus the
> rotator, mast, and upper two antennas and coax were supported by the bottom
> rotator plate.
>
> I next attached the back-to-back (face-to-face?) muffler clamp assembly
> to the mast such that it rested snuggly against the top bearing. We hauled
> up the Bencher Skyhawk and attached it to the mast about a foot above the
> tower top. This weight (75# or so) was now supported by the top plate. The
> combined weight of this mast/antennas/coax is thus distributed across two
> plates, well below the #300 limit of either one. If I ever choose to climb
> the mast, I'd add a third clamp assembly to rest against the middle bearing
> plate and gain a bit more safety margin for my 200-pound frame.
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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